# Proposed shared library: `ZB.MOM.WW.Audit`

A contract on paper — the public surface to extract so the three projects stop
re-implementing audit-event capture with incompatible shapes. Realizes
[`../spec/SPEC.md`](../spec/SPEC.md).
**Not yet created.** Reference implementations already exist: ScadaBridge's
`IAuditWriter`/`IAuditPayloadFilter` (already at target shape), mxaccessgw
structured-log audit trail, OtOpcUa admin-UI audit log.

## Package (.NET 10)

```
ZB.MOM.WW.Audit   # the single package: event record, seams, helpers, DI wiring
```

Single package, single DLL. Only non-BCL dependency:
`Microsoft.Extensions.DependencyInjection.Abstractions` (for `AddZbAudit`).
Published to the Gitea NuGet feed; SemVer.

| Package (→ DLL) | Transitive deps | OtOpcUa | mxaccessgw | ScadaBridge |
|---|---|---|---|---|
| `ZB.MOM.WW.Audit` | `Microsoft.Extensions.DependencyInjection.Abstractions` | ✅ | ✅ | ✅ |

All three auth-bearing processes are .NET 10 — the x86/net48 mxaccessgw worker does
no audit emission, so net48 multi-targeting is **not** required.

## `AuditEvent` record and `AuditOutcome` enum

```csharp
public sealed record AuditEvent {
    public required Guid EventId { get; init; }
    public required DateTimeOffset OccurredAtUtc { get; init; }   // normalized to UTC on assignment
    public required string Actor { get; init; }
    public required string Action { get; init; }
    public required AuditOutcome Outcome { get; init; }
    public string? Category { get; init; }
    public string? Target { get; init; }
    public string? SourceNode { get; init; }
    public Guid? CorrelationId { get; init; }
    public string? DetailsJson { get; init; }
}

public enum AuditOutcome { Success, Failure, Denied }
```

`OccurredAtUtc` is the only field with a normalization contract: any value assigned
is coerced to UTC (via `ToUniversalTime()`). All other fields are caller-supplied and
carried through without transformation by the library internals.

## Seams

### `IAuditWriter`

```csharp
public interface IAuditWriter
{
    Task WriteAsync(AuditEvent evt, CancellationToken ct = default);
}
```

**Hard contract:**
- Best-effort delivery. The implementation **MUST swallow all internal failures** and
  **MUST NOT throw** to the caller. A write that fails silently is preferable to
  a write that crashes the calling thread or kills a request pipeline.
- `CancellationToken` is respected for cooperative cancellation but a cancellation
  does not constitute a contract violation; the implementation may choose to complete
  a partially-written event anyway.

### `IAuditRedactor`

```csharp
public interface IAuditRedactor
{
    AuditEvent Apply(AuditEvent rawEvent);
}
```

**Hard contract:**
- Pure function (no I/O, no side effects).
- **MUST NOT throw.** On any internal failure the implementation must over-redact
  (e.g. replace the affected field with a sentinel such as `"[redacted]"`) rather
  than propagate the exception. Lossier output is always preferable to a thrown
  exception reaching the caller.

## Shipped helpers (concrete)

### Redactors

| Type | Behaviour |
|---|---|
| `NullAuditRedactor` | Identity — returns the event unchanged. Registered as the default by `AddZbAudit`. |
| `TruncatingAuditRedactor` | Caps `DetailsJson` and `Target` to a configurable maximum length and appends a marker (e.g. `"…"`) when truncated. Never throws. Configured via `TruncatingAuditRedactorOptions`. |
| `TruncatingAuditRedactorOptions` | Options record for `TruncatingAuditRedactor`: `MaxDetailsJsonLength`, `MaxTargetLength`, `TruncationMarker`. |

### Writers

| Type | Behaviour |
|---|---|
| `NoOpAuditWriter` | Discards every event. Registered as the default by `AddZbAudit`; consumer replaces with a real writer. |
| `CompositeAuditWriter` | Fan-out: forwards each event to an ordered list of inner `IAuditWriter` instances. A failing inner writer is swallowed (per the `IAuditWriter` contract) — it does **not** abort the remaining writers in the list. |
| `RedactingAuditWriter` | Decorator: calls `IAuditRedactor.Apply` on the event, then delegates the redacted event to an inner `IAuditWriter`. Separates the redaction concern from any concrete writer. |

## DI wiring

```csharp
public static IServiceCollection AddZbAudit(this IServiceCollection services);
```

Registers defaults via `TryAdd` so any prior consumer registration wins:

- `IAuditRedactor` → `NullAuditRedactor` (singleton)
- `IAuditWriter` → `NoOpAuditWriter` (singleton)

A consumer that registers its own `IAuditWriter` (e.g. a Serilog-backed writer or a
`CompositeAuditWriter`) before or after calling `AddZbAudit` will see its registration
respected. `AddZbAudit` does **not** clear or override existing registrations.

## Relationship to Telemetry (`ILogRedactor`)

`IAuditRedactor` mirrors Telemetry.Serilog's `ILogRedactor` in shape and naming — same
single-method contract, same "pure, must not throw, over-redact on failure" semantics —
so that a future `ZB.MOM.WW.Hosting` aggregator package can wire both behind a single
configuration surface without an impedance mismatch.

`ZB.MOM.WW.Audit` has **no dependency** on `ZB.MOM.WW.Telemetry` or any Serilog package.
The alignment is intentional design convergence; the independence is a hard boundary.

## What stays in each consumer

OtOpcUa: admin-UI audit sink (Blazor event handler → `IAuditWriter`), `Category`
constants specific to OPC UA operations.

mxaccessgw: gRPC interceptor that captures actor/action from call metadata; constraint-aware
`Category` tagging; `DetailsJson` serialization of gateway-specific payloads.

ScadaBridge: site-scoped `SourceNode` population; `ManagementActor` enforcement callbacks;
`IAuditPayloadFilter` → `IAuditRedactor` migration (shape is already equivalent — adoption
is a near-zero-effort rename).

## Open contract questions

1. **Batching**: a `WriteBatchAsync(IEnumerable<AuditEvent>, CancellationToken)` overload on
   `IAuditWriter` may be warranted once a database-backed writer is in use. Defer until
   the first consumer demonstrates the need; batching can be added without breaking the
   existing single-event surface.
2. **Structured `DetailsJson`**: confirm whether callers should supply raw JSON strings or
   whether a typed `TDetails` generic overload (serialized internally) is cleaner. The
   current `string?` keeps the library dependency-free but shifts serialization to the caller.
3. **`CompositeAuditWriter` error policy**: decide whether per-writer failure should be
   observable (e.g. an optional `ILogger<CompositeAuditWriter>`) or always silently dropped.
   Logging the failure is diagnostic-friendly but adds a logging dependency.

See [`../GAPS.md`](../GAPS.md) for the adoption order and effort/risk.
